Recent 50 MHz E region coherent backscatter observations and in situ rocket measurements established the existence of enhanced electric fields in the midlatitude ionosphere that can become at times sufficiently large to excite the Farley-Buneman instability. To understand the origin of these fields, we present a simple quantitative model that relates to a local polarization process acting inside spatially confined, nighttime sporadic E layers of dense ionization. By including the effects of field-aligned currents in the current continuity equation we estimate the necessary conditions on the relative horizontal E layer extent and the ratio of integrated Pedersen conductivities above and inside the layer for the generation of both zonal and meridional polarization fields. We show that the polarization process can account for the elevated electric fields of several millivolts per meter, which are implied often from backscatter Doppler measurements during unstable E region conditions at midlatitude. The polarization process can become much more effective for dense and strongly elongated Es layers under the action of an enhanced ambient electric field. In this case, large polarization fields that may be capable of exciting Farley-Buneman plasma waves can be sustained. The stringent requirements for strongly elongated sporadic E layers with sharp boundaries, low ionospheric Pedersen conductivities above the layer in relation to those inside, and relatively large ambient electric fields would explain why type 1 echoes are so rare in midlatitude E region backscatter. ¿ 1998 American Geophysical Union